Field of the Invention
The present invention relates to an acoustic signal generator, in particular a horn, and to a method for generating an acoustic signal. The acoustic signal generator has a membrane that can oscillate, a deflection sensor for detecting any deflection of the membrane, and an exciter configuration coupled to the membrane.
Acoustic signal generators of this generic type have a membrane that can oscillate, is normally composed of metal, and is coupled to the exciter configuration. The exciter configuration normally has an exciter coil and an armature that is inductively coupled to the exciter coil and is connected to the membrane. In known appliances, a mechanical switch is provided for applying a supply voltage to the exciter winding, with the armature together with the membrane being deflected when the switch is closed, and current thus flows through the coil, and with the membrane together with the armature moving back again in the direction of its original position when the switch is subsequently opened, and overshooting beyond the original position. The mechanical switch is coupled to the membrane and is opened again when the membrane has reached a specific deflection when the switch is closed, the deflection being dependent on the configuration of the mechanical switch on the membrane. The mechanical switch is opened and closed in a clocked manner in this way, with the clock frequency being dependent on the natural frequency of the oscillating system that contains the membrane and armature. The membrane in consequence oscillates at its natural frequency, which is in the human audibility range in the case of horns.
The volume can be adjusted by the configuration of the mechanical switch on the membrane, with the tone which is generated being quieter when the switch is switched off again while the membrane deflection is still small, and with the tone which is generated being louder when the mechanical switch is not switched off again until the membrane deflection is greater.
A configuration such as this has the disadvantage that spark emissions can occur at the mechanical switch when the exciter winding is disconnected from the supply voltage and, in some circumstances, this results in severe electromagnetic radiated interference emission.
Furthermore, a considerable power loss occurs in an uncontrolled manner in the switch, which is driven in a clocked manner at the natural frequency of the oscillating system containing the membrane and armature, which is normally several hundred Hertz, and this can considerably reduce the life of known horns.
It is accordingly an object of the invention to provide an acoustic signal generator, and a method for generating an acoustic signal which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an acoustic signal generator. The signal generator has a membrane which can oscillate, a deflection sensor for detecting any deflection of the membrane, an exciter configuration coupled to the membrane, a power semiconductor switch having a load path connected to the exciter configuration and a drive connection, and a drive circuit having a first connection connected to the drive connection of the power semiconductor switch and generating a drive signal available at the drive connection. The drive circuit has a second connection connected to the deflection sensor.
Accordingly, the acoustic signal generator according to the invention has, in addition to the membrane which can oscillate, the deflection sensor, and the exciter configuration which is coupled to the membrane, a power semiconductor switch and a drive circuit which is connected to a drive connection of the power semiconductor switch and to which the deflection sensor is connected.
The exciter configuration preferably contains an exciter winding and an armature which is inductively coupled to the exciter winding, with the exciter winding being connected to a supply voltage in series with a load path of the power semiconductor switch. The use of a power semiconductor switch, in particular of a power MOSFET has the advantage over the use of a mechanical switch for switching the exciter winding that the electromagnetic interference emissions that occur during switching are considerably reduced.
The semiconductor switch that is used is preferably a temperature-protected semiconductor switch that is marketed, for example, by Infineon Technologies AG, Munich, under the designation TEMPFET. Ideally, the semiconductor switch has, in addition to temperature protection, integrated overvoltage protection and/or integrated short-circuit protection, and semiconductor switches such as these are marketed by Infineon Technologies AG, Munich, under the designation HITFET. Temperature-protected semiconductor switches protect themselves and switch themselves off when their temperature exceeds a predetermined value owing to the power losses that occur. The temperature-protected semiconductor switch is preferably thermally coupled to the housing in which the exciter configuration is accommodated. In this way, the semiconductor switch also monitors the temperature in the vicinity of the exciter configuration and switches itself off, and cannot be switched on, when the temperature is above a predetermined value. This measure contributes to increasing the life of the signal generator since this prevents the exciter coil from being overheated.
The switch-on resistance of the semiconductor switch is preferably selected such that a not inconsiderable proportion of the total power loss that occurs is incurred in the semiconductor switch. The power loss in the exciter winding is reduced by this measure, which likewise contributes to increasing the life of the signal generator.
The deflection sensor, which is connected to the drive circuit, is preferably a capacitive sensor that has at least one capacitor, whose capacitance varies as a function of the deflection of the membrane. The capacitance of this at least one capacitor is evaluated in the drive circuit, with the power semiconductor switch always being opened when the capacitance is greater than or less than a predetermined value. Various known evaluation circuits may be used to determine the capacitance of the variable capacitor. For example, one embodiment of the invention provides for the capacitor to be connected in series with a current source and for the current from the power source to be applied to the capacitor for a predetermined time period, and for the voltage that is present across the capacitor to be measured at the end of this time period. In this case, use is made of the fact that the voltage that is produced on the capacitor by the charge flowing into it is proportional to its capacitance, given that the charging current and the charging time are the same.
A further embodiment provides for the capacitor to be charged to a predetermined voltage, and for the change in the voltage across the capacitor to be observed. The charge that is stored in the capacitor in this case remains constant, so that the voltage across the capacitor rises when its capacitance decreases, and vice versa.
A further embodiment provides for the capacitor to be connected in a first series resonant circuit of a bridge circuit, with the bridge circuit having a second series resonant circuit in parallel with the first series resonant circuit, and with the two series resonant circuits being excited by an AC voltage. The frequency of the first series resonant voltage in this case varies with the value of the capacitance of the capacitor in the capacitive sensor. The two series resonant circuits each have a tapping point for tapping off a potential in the respective series resonant circuit, with the tapping points being connected to an evaluation circuit which uses the difference between these two potentials to produce a drive signal, which is dependent on the value of the capacitance of the variable capacitor, for the semiconductor switch. The drive circuit evaluates, in particular, the zero crossing of the difference voltage, with the components in the bridge circuit being matched to one another such that, at the zero crossing of the difference signal, the variable capacitor has a capacitance which results in the membrane reaching that deflection at which the switch is intended to be switched off. The bridge circuit is used to trim the capacitance of the variable capacitor to a nominal value, which is dependent on the other components in the bridge circuit.
In order to provide the capacitive sensor, a first embodiment of the invention provides for a first capacitor plate of the at least one capacitor in the capacitive sensor to be formed by the membrane itself. A further embodiment provides for the first capacitor plate to be formed by a first electrode, which is mechanically coupled to the membrane or to the armature. The first electrode is in this case deflected in the same way as the membrane.
A second capacitor plate of the at least one capacitor in the capacitive sensor is, according to one embodiment of the invention, formed by a housing which surrounds the membrane and, possibly, the exciter configuration and is electrically insulated from the membrane. A further embodiment provides for the second capacitor plate to be formed by a second electrode, which is disposed such that it is at a distance from the membrane and is insulated from the housing. The second capacitor plate can also be formed by a housing cover disposed above the membrane.
The membrane or the first electrode, which forms the first capacitor plate, and the housing, the second electrode or the cover, which forms the second capacitor plate, have suitable connections for connection to the drive circuit.
In exemplary embodiments in which the membrane is not composed of metal, the invention provides for metal to be vapor-deposited onto a portion of the membrane, in order to form the first capacitor plate.
In accordance with an added feature of the invention, the drive circuit has a third connection for receiving a switch-on signal.
In accordance with another feature of the invention, the drive signal is dependent on a capacitance of the capacitor of the capacitive sensor.
In accordance with a further feature of the invention, the drive circuit has a current source, a drive circuit switch connected in parallel with the capacitor, and a comparator circuit connected to the capacitor for evaluating a capacitance of the capacitor. The current source is connected in series with the capacitor. The comparator circuit compares a voltage across the capacitor with a reference voltage, and, the comparator circuit has an output providing an output signal that is dependent on a comparison.
In accordance with an additional feature of the invention, the drive signal is dependent on the output signal at the output of the comparator circuit, and on the switch-on signal.
In accordance with another further feature of the invention, the drive circuit has a diode connected in series with the capacitor, a drive circuit switch connected in parallel with the capacitor, and a comparator configuration connected to the capacitor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for generating an acoustic signal in dependence on a switch-on signal. The method includes providing a membrane which can oscillate, an exciter configuration coupled to the membrane, a drive circuit receiving the switch-on signal, a power semiconductor switch connected to the drive circuit, and a deflection sensor for detecting any deflection of the membrane. An opening and closing of the power semiconductor switch is clocked for as long as the switch-on signal is at a given value, with a closing duration, during which the power semiconductor switch is closed during a clock period, being dependent on the deflection sensor.
In accordance with an added mode of the invention, there is the step of forming the deflection sensor as a capacitive sensor having at least one variable capacitor, and in which the closing duration is dependent on a capacitance of the variable capacitor.
In accordance with another mode of the invention, there is the step of determining a value of the capacitance of the variable capacitor when the power semiconductor switch is opened and after the switch-on signal has assumed the given value, and the value of the capacitance of the variable capacitor is taken into account when determining the closing duration of the power semiconductor switch.
In accordance with a concomitant feature of the invention, there is the step of opening the power semiconductor switch again after being closed, when the capacitance of the variable capacitor has changed by a predetermined percentage value.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an acoustic signal generator, and a method for generating an acoustic signal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.